The mechanism of rat thrombocytopenia induced by i.v. injections of platelet-activating factor (PAF-acether) was investigated. Platelet counts performed after diluting the blood samples in 1% formalin in saline showed that PAF-acether (6 micrograms/kg i.v.) induced a significant thrombocytopenia in rats, which peaked within 1 h, followed by a drastic increase of platelet counts at 4 h and a return to basal levels at 24 h. At this time, it was not possible to induce thrombocytopenia with a second challenge with PAF-acether, indicating a clear state of desensitization which disappeared within five days after the first injection of PAF-acether. The pretreatment with the specific PAF-acether receptor antagonist, BN 52021 (2.5-15 mg/kg), 48740 RP (6-25 mg/kg) and WEB 2086 (0.25-1 mg/kg) blocked the thrombocytopenia dose dependently. The lipoxygenase inhibitor nordihydroguaiaretic acid, at 25-100 mg/kg, was also effective against the thrombocytopenia induced by PAF-acether, reinforcing the potential involvement of arachidonic acid derivatives in this process. Adrenal hormones may modulate this process, since adrenalectomized animals responded to PAF-acether with exacerbated thrombocytopenia. No reduction in the platelet counts was noted when the blood was diluted in formalin-free saline, indicating that unstable aggregates were formed in vivo, which tended to resolve in vitro. Our results suggest that the thrombocytopenia induced in rats by PAF results from a reversible process of intravascular platelet aggregation, probably following the secretion of platelet-activating substances released by a first-hit blood cell.